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IC 


8968 



Bureau of Mines information Circular/1984 



r 




Performance Evaluation of a Real-Time 
Aerosol Monitor 

By K. L. Williams and R. J. Timko 



??^ 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 8968 

Performance Evaluation of a Real-Time 
Aerosol Monitor 

By K. L. Williams and R. J. Timko 




UNITED STATES DEPARTMENT OF THE INTERIOR 
William P. Clark, Secretary 

BUREAU OF MINES 
Robert C. Norton, Director 



{\0' 



Library of Congress Cataloging in Publication Data: 



Williams, Kenneth L., 1952- 

Performance evaluation of a real-time aerosol monitor. 

(Information circular; 8968) 

Bibliography: p. 16. 

Supt. of Docs, no.: I 28.27:8968. 

1, Mine dusts— Measurement— Instruments. 2. Aerosols— Measure- 
ment— Instruments. I. Timko, Robert J. II. Title. III. Title: Perform- 
ance evaluation of the RAM-1. IV. Series: Information circular (United 
States. Bureau of Mines) ; 8968. 



TN295.U4 [TN312] 622s [622'.8] 83-600363 



^. CONTENTS 

U^ Page 



§ 



b 



Abstract 1 

^ Introduction 2 

^ Equipment and procedure 2 

Test chamber 2 

"^(^y^ Llppmann-type sampler arrangement 3 

^ Gravimetric samplers i 3 

^, The RAM-1 3 

Tubing dust losses 5 

^\^~" Test procedures 6 

Results and discussion 6 

Uncertainty In the gravimetric measurement 7 

Linearity of RAM-1 response 9 

Effect of particle characteristics 10 

Size or density 11 

Shape factor 12 

Surface properties 13 

Calibration 13 

RAM-1 precision 13 

Conclusions 15 

Recommendations 16 

References. 16 

Appendix A. — Dust losses In tubing 17 

Appendix B, — Recommendations on the calibration of the RAM-1 20 

ILLUSTRATIONS 

1. Test setup 3 

2. Llppmann-type sampler 3 

3. Real-time aerosol monitor 4 

4 . Standard and modified cyclone arrangements 5 

5. Comparison of resplrable dust concentrations measured by RAM-1 with con- 
centrations measured gravlmetrlcally for coal 1 7 

6. Comparison of resplrable dust concentrations measured by RAM-1 with con- 
centrations measured gravlmetrlcally for coal 2 8 

7. Comparison of resplrable dust concentrations measured by RAM-1 with con- 
centrations measured gravlmetrlcally for Arizona Road Dust 8 

8. Comparison of resplrable dust concentrations measured by RAM-1 with con- 
centrations measured gravlmetrlcally for limestone 1 9 

9. Comparison of resplrable dust concentrations measured by RAM-1 with con- 
centrations measured gravlmetrlcally for limestone 2 9 

Response of RAM-1 to coal 1, coal 2, limestone 1, limestone 2, and Arl- 

~^ zona Road Dust....... 11 

11. Scanning electron microscope micrograph of coal 1 12 

-^ 12. Scanning electron microscope micrograph of coal 2 13 

c^A-1. Comparison of resplrable dust concentrations measured with filters Inside 
_d the dust chamber with concentrations measured with filters outside the 

dust chamber 17 

A-2. Comparison of resplrable coal dust concentrations measured by RAM-1 with 
concentrations measured gravlmetrlcally inside and outside the dust 

chamber 19 



iio. 



ii 



TABLES 

Page 

1. Regression statistics 10 

2. Dust parameters 12 

3. Ninety-five-percent confidence intervals at Yc = 2.0 mg/m^ 15 





UNIT OF MEASURE ABBREVIATIONS USED 


IN 


THIS REPORT 


cm 


centimeter 


m^/min 




cubic meter per minute 


ft 


foot 


mg 




milligram 


ft3/cm 


cubic foot per minute 


mg/m^ 




milligram per cubic meter 


ga 


gauge 


min 




minute 


gal 


gallon 


mm 




millimeter 


g/cm3 


gram per cubic centimeter 


ym 




micrometer 


h 


hour 


pet 




percent 


L/min 


liter per minute 


yr 




year 



PERFORMANCE EVALUATION OF A REAL-TIME AEROSOL MONITOR 

By K. L. Williams ^ and R. J. Timko2 



ABSTRACT 

The Bureau of Mines laboratory tested the response of a commercially 
available real-time aerosol monitor (GCA RAM-1) to various dusts. Mon- 
itor measurements were recorded, averaged, and compared with simultane- 
ous gravimetric measurements of each test dust. Tests usually lasted 
several hours. The test dusts of various particle size distributions 
used included coal, limestone, and a commercially available test dust. 
For each particular dust, the monitor response was linear and corre- 
lated well with mass concentration over the range of about 0.5 to 10 
mg/m^ . 

The monitor can estimate a 2.0-mg/m^ respirable coal dust concentra- 
tion within as little as ±6 pet with 95 pet confidence. The monitor 
must, however, be calibrated with the dust to be measured because the 
instrxament response is affected by the type of dust particle. The 
average monitor response to a mass concentration of coal dust was ap- 
proximately twice the average monitor response to the same mass concen- 
tration of limestone dust. 



1 Physicist, 
^physical scientist. 
Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 



INTRODUCTION 



In 1978, GCACorp., Technology Div. , 
Bedford, MA, designed and fabricated the 
RAM-1 for the Bureau of Mines under con- 
tract H0377092, "Improved Light Scatter- 
ing Dust Monitor." This report describes 
the procedure and results of laboratory 
tests performed by the Bureau to evaluate 
the response of the RAM-1 to various 
dusts. 

The RAM-1 measures resplrable dust con- 
centrations In the air almost Instantane- 
ously by using a light-scattering tech- 
nique. A sampling pump draws air Into a 
sensing chamber where It passes through 
the path of a light beam. Dust In the 
air scatters some of the light to a de- 
tector that produces an electrical signal 
proportional to the Intensity of the 
light. 

The Intensity of light scattered from 
dust particles Is a function of the par- 
ticle size, shape, refractive Index, 
wavelength of the light, and the angle 
from which the light Is detected. It Is 
not a function of the density of the par- 
ticles , and thus Is not a direct function 
of the mass of the particles. 



dust. For that reason, GCA tried to de- 
sign the RAM-1 to measure the mass con- 
centration of dust, regardless of parti- 
cle characteristics such as size, shape, 
or refractive Index, GCA selected the 
design based on a dust monitor built ear- 
lier In the Federal Republic of Germany 
with similar characteristics. That moni- 
tor (the Tyndallometer TM-Dlgltal) (O^ 
successfully reduced the dependence of 
light scattering on the characteristics 
of particles In coal mines. 

The RAM-1, because of Its fast re- 
sponse, could be used In many mining ap- 
plications, especially for dust monitor- 
ing for control of resplrable coal mine 
dust (2^). Thus, the Bureau tested the 
response behavior of the Instrument when 
measuring various test dusts. The RAM-1 
was treated as a "black box;" no prior 
assumptions were made about response be- 
havior. The questions were — 



1. Is the Instrument response 
with mass concentration? 



linear 



2. Does the Instrument respond differ- 
ently to different dusts? 



However, the health hazards posed by 
various dusts In mines are related to the 
mass concentration of the particular 



3. What Is the estimate 
for a given type of dust? 



of precision 



EQUIPMENT AND PROCEDURE 



Both the RAM-1 's and gravimetric dust 
sampling devices were exposed to various 
concentrations and particle size distri- 
butions of coal dust, limestone dust,'* 
and Arizona Road Dust^ (ARD).^ A de- 
scription of the equipment, test proce- 
dures, and rationale follows. 

TEST CHAMBER 

Figure 1 Illustrates the test setup. 
The test dust was dispersed by feeding 

■^Underlined numbers in parentheses re- 
fer to items in the list of references 
preceding the appendixes. 

^Typically called "rock dust," lime- 
stone dust is mixed with dust on surfaces 
inside coal mines to reduce f lammability. 



the dry dust Into a 61- by 91- by 122-cm 
plywood chamber using either a Wright 
dust feeder or a TSI model 3400 fluld- 
Ized bed aerosol generator (FBAG) O). 
Although the FBAG characteristically pro- 
duced a more constant dust concentration 
In the chamber, the Wright dust feeder 
was occasionally needed to obtain higher 
concentrations. Inside the chamber, a 
small fan was used to mix the dust and 
air. 

^Reference to specific products does 
not imply endorsement by the Bureau of 
Mines. 

^ARD is a carefully sized, commercial 
test dust used primarily to test the ef- 
ficiency of air filters for internal com- 
bustion engines. 



V\\ VVV\V\VVVVVlV\lkkV 



; J= 61 cm TOP VIEW 



SIDE VIEW 




^To vacuum source 
<-- — lO-port, 2-L/min critical 
orifice monifold 



Stack sampler 

to 14.2-L/mm 

vacuum source 



RAM-Is 



FIGURE 1, . Test setup. 

LIPPMANN-TYPE SAMPLER ARRANGEMENT 

Experience had indicated that the small 
fan ensured that no severe concentra- 
tion gradients existed in the chamber — at 
least when measurements were averaged 
over several hours. Nevertheless, to as- 
sure that all dust sampling devices were 
exposed to as uniform a dust cloud as 
possible, a device patterned after the 
Lippmann sampler (4^) was used. 

The Lippmann-type sampler (fig. 2) was 
fabricated from a 5-gal metal can. Ap- 
proximately 18 cm from the top, a round 
plexiglass platform was mounted in the 
can with 20 holes drilled equidistant at 
a 10-cm radius. Dorr-Oliver cyclones (5- 
6^) (sampling heads) were placed in those 
holes with the inlets facing the center. 
A lid with a 5-cm-diam hole cut in the 
center was placed on top of the can. 



3! cm 



Sampling positions 




SIDE VIEW TOP VIEW 

FIGURE 2. • Lippmann-type sampler. 



Dusty air in the chamber entered the 
device through the hole in the lid and 
was drawn vertically downward toward the 
center of the plexiglass platform. Be- 
cause all cyclones mounted in the plat- 
form operated at 2-L/min flow rate, the 
dust cloud was distributed evenly along 
each sampling cyclone. With this ar- 
rangement, the coefficient of variation 
among the 10 gravimetric sampling device 
measurements was almost always less than 
10 pet. 

GRAVIMETRIC SAMPLERS 

Ten gravimetric sampling devices were 
used to make a reference measurement of 
the mass concentration of dust. Each 
gravimetric device consisted of a Mine 
Safety Appliance Co. (MSA) sampling head 
connected to a critical orifice (7^) con- 
trolled airflow system. MSA sampling 
heads consist of Dorr-Oliver 10-mm-diam 
nylon cyclone particle size classifiers 
and preweighed 37-mm polyvinylchloride 
membrane filters in plastic filter cas- 
settes. The cyclones retain the large, 
nonrespirable particles and allow respi- 
rable particles to pass through to the 
filters. 

For the tests, critical orifices (CO's) 
were fabricated from 20-ga hypodermic 
needles. The orifices were calibrated to 
2±0.01 L/min by shortening the needle or 
modifying the opening. A wet test meter 
was used to measure airflow through the 
CO's. Initially, flow rates were checked 
before and after each test. Later, be- 
cause the CO's were quite stable, spot 
checks were relied on to assure a con- 
stant 2-L/min flow rate through the grav- 
imetric devices. 

The components for each gravimetric 
sampling device were labeled and always 
used together. That procedure helped in 
the location and correction of any equip- 
ment problems. 

THE RAM-1 

The RAM-1 (fig. 3) measures and dis- 
plays current respirable dust levels in 




FIGURE 3. - Real-time aerosol monitor (RAM-1). 



the air. The dusty air is drawn through 
a lO-mm-diam Dorr-Oliver nylon cyclone, 
Respirable dust (8^) , the portion of the 
sampled dust that passes through the cy- 
clone, enters a light-scattering chamber. 
Here the instrument detects light scat- 
tered from the particles at an angle 
of 70°±25°, The detector converts the 
light into an electrical signal propor- 
tional to the amount of dust present in 
the airstream, A detailed description of 
the instriiment design has been given in 
other reports U, 9-10). 

Circuitry is temperature compensated 
and protected against humidity. The Bu- 
reau extensively tested the electronic 
characteristics of the RAM-1 and found 



the instrument to be extremely stable. 
Those test results are discussed in the 
contract final report (9^) . 

The RAM-1 's also sampled from inside 
the Lippmann-type sampler. For the RAM- 
I'S, MSA sampling heads were modified as 
follows (fig, 4), The fitting from the 
cyclone holding bracket normally used to 
connect the flexible tubing to the exit 
side of the filter cassette was removed, 
A short copper sleeve was inserted as a 
spacer between the top of the bracket and 
the top of the nylon cyclone. The spacer 
kept the cyclone vortex finder properly 
aligned and sealed to the cyclone body. 
Flexible tubing used to connect each 
RAM-1 to its sampling head was inserted 



ji^^Or^^^f^ r^T^ 



Adjustable 
knob 




Flexible 
plastic tubing 



To 2-L/min 
vacuum source 



Adjustable 
knob 



Cyclone 




Standard Modified 

FIGURE 4. - Standard (left) and modified (right) cyclone arrangements. 



through the top of the bracket, passed 
through the copper sleeve, and then was 
connected to the exit port of the 
cyclone. 

Resplrable dust passing through the cy- 
clone flowed through the flexible tubing 
to the entry port of the RAM-1 located 
outside the chamber. Tubing between the 
sampling head and the RAM-1 units was 
limited to 3-ft lengths to minimize dust 
losses in the tubing. 

TUBING DUST LOSSES 

Dust losses can occur when transport- 
ing dusty air through tubing. Mounting 



of the cyclone on the RAM-1 inlet and 
placement of the entire instrument with- 
in the test chamber could reduce these 
losses. This was not done in the Bu- 
reau's tests; however, the test setup 
used was justified in two ways: First, 
such an arrangement allowed use of the 
Lippmann-type sampler so that all sam- 
pling heads were exposed to the same dust 
cloud^ and, second, dust losses in the 

^Because of the volume occupied by t±ie 
bodies of the RAM-1 's, the cyclone inlets 
could not be placed near enough to each 
other to ensure each instrument sampled 
the same dust atmosphere. 



tubing were assximed to be about the same 
In all cases. 

A short series of tests was performed 
to investigate the magnitude and constan- 
cy of dust losses in the tubing. A dis- 
cussion of these tests and the results 
are given in appendix A. Briefly, dust 
losses in the tubing were less than 10 
pet. Because the purpose of the evalua- 
tion was to compare responses of the 
RAM-1 to various dusts, not to calibrate 
the instruments to indicate an absolute 
value, that small constant bias was not 
troublesome. 

However, dust losses were not constant. 
Variations in dust losses appear as 
random error of the RAM-1 measurements, 
causing the RAM-1 measurements to appear 
to be less precise. If the RAM-1 was 
operated with the cyclone mounted direct- 
ly on the inlet, variable tubing losses 
would be eliminated, and the RAM-1 preci- 
sion would be slightly higher. 

TEST PROCEDURES 

The RAM-1 units were operated from 
their battery chargers to avoid problems 
with battery discharge or failure. With 
the RAM-1 reference scatterer inserted 
into the light beam, the gain of each 
RAM-1 unit was adjusted so that the in- 
strument indicated the calibration value 
recommended by the manufacturer. Be- 
fore each test, the zero and gain were 
checked. It was found that adjustments 
to gain and zero were rarely needed. 

Once the RAM-1 units and gravimetric 
sampling units were prepared, the dust 
generation system was started and allowed 
to stabilize. Determination of the dust 
concentration stabilization (usually af- 
ter 1 h) was made by observing readings 
on the RAM-1 units. The flow system for 
the gravimetric sampling devices was then 
turned on and recording of the electrical 



output signal from the RAM-1 units on a 
strip chart recorder began. Tests lasted 
approximately 4 h, depending on the con- 
centration of the test dust. 

The objective when deciding test dura- 
tion was to sample long enough to collect 
at least 1 mg of dust on the filters of 
the gravimetric sampling devices. The 
precision of the analytical balance for a 
single weighing was ±0.01 mg. Weighing 
precision, when considering preweighing 
and postweighing necessary to determine 
the mass of the collected dust, was then 
0.014 mg. With a dust mass of at least 
1 mg, relative weighing error attributa- 
ble to the balance, neglecting any error 
introduced by the operator, was limited 
to 1.4 pet. 

After the test, the area under the 
curve of each RAM-1 recorded output trace 
was calculated to determine the average 
reading over the test period. That read- 
ing was compared with the mean of the 10 
gravimetric concentration measurements. 
The gravimetric concentration (Cone.) was 
determined using the following equation: 



Cone, (mg/m^) = 



3^ = 



Am 



(0.002) (t) 



(1) 



where Am = mass of the dust collected 
on the filter, mg. 



and 



t = sampling time, min. 



The constant, 0.002, is the flow rate of 
the samplers in cubic meters per minute. 

Periodically throughout the series of 
tests, the size distribution of the test 
dust was measured with an Andersen Mark 
III stack sampler. The Mark III is an 
eight-stage cascade impactor with stage 
size cutoffs ranging from 13.6 to 0.54 pm 
when operated at 14.2 L/min (0.5 ft^/ 
min) . 



RESULTS AND DISCUSSION 



The intensity of light scattered from 
dust particles is not a direct function 
of the mass of the particles. Light- 
scattering theory (12) states that the 



intensity of the light scattered by a 
particle will depend on such things as 
the wavelength of the source light , the 
angle between the incident and the 



detected light, particle index of refrac- 
tion, and particle size. It does not 
depend on particle density. Light- 
scattering behavior is fairly predictable 
for ideal spherical particles. However, 
in most situations, particle shape fac- 
tors complicate the matter. Thus, the 
theory does not predict a direct func- 
tional relationship between the intensity 
of scattered light and the mass of the 
particle. Any correlation between the 
intensity of the scattered light and the 
mass concentration of dust is statistical 
rather than functional. No exact mathe- 
matical relationship exists that relates 
scattered light intensity to particle 
mass in all cases. For a more complete 
discussion of statistical versus func- 
tional relationships, see reference 13. 

UNCERTAINTY IN THE GRAVIMETRIC 
MEASUREMENT 

Figures 5 through 9 show the scatter 
plots and linear regressions of RAM-1 
readings versus gravimetrically deter- 
mined respirable dust concentrations. 
Each data pair (x, y) represents the 
RAM-1 reading averaged over the time of 
one test (y value) and the mean of 10 
gravimetric measurements obtained during 
the same time period (x value) . 

The root-mean-square estimated relative 
standard devi ati on of the gravimetric 
measurements (RSD) for all tests is 0.11. 
This value was calculated as follows: 




E 



RAM-1 -- 1.08 gravimetric +0.55 .o 




12 3 4 

GRAVIMETRIC MEAN, mg/m^ 

FIGURE 5, - Comparison of respirable dust concen- 
trations measured by RAM-1 with concentrations mea- 
sured gravimetrically for coal 1. 



expected uncertainty in a single measure- 
ment made by one gravimetric sai]q>ler. 



RSD = [^I(S,/X,)2] ' ', (2) 

where N = the number of tests, 

S| = the estimated standard devi- 
ation of the gravimetric 
measurements for the ith 
test. 



The independent variable plotted in 
figures 5 through 9, however, is not a 
single gravimetric measurement. Instead, 
the mean (X) of 10 such measurements was 
used to estimate the mass concentration. 
To determine how well this mean estimates 
the true concentration of dust (mq^ ^^ 
measured by gravimetric samplers, the 
following expression is used: 



and 



X| = the mean of the gravimetric 
measurements for the ith 
test. 



Obviously, not every test exhibits a RSD 
of exactly 0.11; however, this root-mean- 
square average could roughly indicate the 



= X ± 



^(v,a) ^* 



(3) 



where v = n-1 = degrees of freedom, 
n = number of samples. 



m 

^ 8 

a> 

E 

(S 6 

Q 

< 
q: 4 



3 2- 



< 
a: 







"^glO 



' 1 1 1 

RAM-1 = 2. 14 gravimetric 
r=0.89 

-Sy.x =0.77 

O 

o 


1 
-0.01 

o 

O y^ 
O/^ 


O 


1 

c 

O 


Oy/^ 


X 


— o 


>^ 


v° 








- 




% 










~ 


^^ go 

'^ \ 1 1 


1 


1 




1 


1 


- 



E 



8- 



5 6 

a: 
CD 4 



^ 2 

I 

^ 

to 8 



E 



1 1 1 1 1 1 1 

- RAIV1-1= 2.50 gravimetric + 0.20 y" 

r=0.95 o ^5^ 

Sy.x =0.66 o/^ ° 


X 


°o/^° 


- 


<9^^ 




5^ 


- 


- ^^ 8 


- 


-""^ 1 


- 



,- 6- 



z 

Q 
< 

cr ^ 
o 



5 2- 



< 



^ r 

RAM-l = 1.73 gravimetric + 1.14 

r = 0.93 o o 

Sy.x = 0.48 

o 




I 2 3 ^ 

GRAVIMETRIC MEAN, mg/m^ 

FIGURE 6. - Comparison of respirable dust concen- 
trations measured by RAM-1 with concentrations mea- 
sured gravimetrically for coal 2, 



and 



1-a = confidence level, 

S = estimated standard deviation 
of the samples. 



In this case, n = 10 and v = 9. From a 
table of values for t(^^c(), it is found 



E 

eT 

z 

Q 
< 
UJ 

q: 



10 
8 
6 
4 
2 



< 



ro 



E 

CD 

Z 
Q 
< 
UJ 
CC 

O 

Z 



< 



— I 1 1 1 I I I r 

RAM-1 = 0.94 gravitnetric +0.57 ^ 
r = 0.95 
■Sy.x=0.65 




q ! I I I I I I 



J L 



10 
8 
6 
4 
2 



— I 1 1 1 1 1 r 

RAM-1 = 0.82 gravimetric + 0.88 
r= 0.97 

•Syx =0.50 




_L 



I 23456789 10 
GRAVIMETRIC MEAN, mg/m^ 

FIGLHRE 7, - Comparison of respirable dust concen- 
trations measured by RAM-1 with concentrations mea- 
sured gravimetrically for Arizona Road Dust, 



that for a confidence level of 95 pet, 
t(9, 0.05) = 2.262. 

As an example, calculate the 95-pct 
confidence interval for the true concen- 
tration using equation 3 when the sample 
mean (X) is equal to 2.0 mg/m^. If RSD 
= 0.11, then a likely estimated standard 
deviation (S) would be 0.22, since S 
= (RSD)X = (0.11)(2.0). Substituting in- 
to equation 3 shows Pq ~ 2.0±0.16 mg/m^ 
~ 2 mg/m^ ±8 pet. In other words, if the 
mean of 10 gravimetric mass measurements 
for a particular test is 2.0 and the es- 
timated standard deviation of one mea- 
surement is 0.22, then a 95-pct assurance 
exists that the true concentration in the 



en 

E 



o 4- 



< 

LU 

cr 
< 



5 2- 



3 
I 

< 



1 1 1 

RAM-1= 0.75 gravimetric 
r = 0.82 

- Sy.x =0.66 


1 

+ 0.56 

o 

o 


1 


^ 


^ 


- 


o 
o ^^ 

y^ o 


%^ 


o 
o 


o 


- 


y^ 


<J3 










y^ 












1 


1 1 


1 


1 








2 3 4 5 6 

GRAVIMETRIC MEAN, mg/m^ 

FIGURE 8« - Comparison of respirable dust concen- 
trations measured by RAM-1 with concentrations mea- 
sured gravimetrically for limestone 1. 



to 


7 


e 




>^ 




o> 


h 


E 




o 


5 


z 




n 




< 


4 


UJ 




a: 




< 


6 


H 




Z 


7 


Z) 




-H 




s 


\ 


< 




a: 





ro 


7 


E 





1 1 1 1 1 
RAM- 1 = 0.99 gravimetric - 0.04 
-r=0.96 . 


y^- 


_Sy.x = 0.49 o^ 


- 


y^ o 


- 


- ° y'^'^ 


- 


cr o 


- 


y^ 1 1 1 1 1 


1 




2 3 4 5 6 7 

GRAVIMETRIC MEAN, mg/m^ 

FIGURE 9. - Comparison of respirable dust concen- 
trations measured by RAM-1 v/ith concentrations mea- 
sured gravimetrically for limestone 2, 



test chamber as measured by the reference 
method is within the interval of 2 mg/m^ 
±8 pet. 

Normally, the independent variable in 
a regression analysis is defined to be 
without uncertainty. From the preceding 
discussion it can_be seen that if the 
gravimetric mean (X) is used to estimate 
the dust concentration in the test cham- 
ber, the uncertainty is not excessive. 
Therefore, it is assumed that using X as 
the independent variable in the regres- 
sion analysis is at least reasonable. 

Note, however, that any estimate of un- 
certainty in the RAM-1 measurements will 
be inflated by the uncertainty in the 
gravimetric reference measurement and re- 
sult in an underestimation of the true 
precision of the RAM-1. 



LINEARITY OF RAM-1 RESPONSE 

By visually reviewing the plots in fig- 
ures 5 through 9, it was concluded that 
the response of the RAM-1 is linear with 
respect to mass concentration for each 
test dust. To verify that conclusion, a 
statistical randomness test was applied 
to examine linearity (11) . Briefly, the 
sequence of signs (+ or -) of the devia- 
tions of the measured y values (RAM-1 
responses) from the corresponding fitted 
regression line values in order of in- 
creasing X values (mean gravimetric mea- 
surement) , was considered. The following 
were determined with this information: 
(a) the number of "+" signs, (b) the num- 
ber of "-" signs, and (c) the number of 
runs. A run is defined as an uninter- 
rupted series of one or more of the 
same sign. Values a and b were used as 



10 



parameters in a table of critical values 
for runs. If the observed number of runs 
was within the two limiting table values, 
then the null hypothesis of linearity 
could not be rejected at the significance 
level of the table elected for use. 

The randomness test for linearity was 
performed on each of the plots in figures 
5 through 9. In no instance, at a sig- 
nificance level of 0.05, could the null 
hypothesis of linearity be rejected; that 
is, it could not be denied that the re- 
sponse of the RAM-1 to the various test 
dusts was linear. 

There are better tests for linearity 
than the above randomness test; however, 
most require that there be more than one 
observed RAM-1 reading (y value) for each 
corresponding dust level (x value) . Un- 
fortunately the dust generation system 
used in this evaluation could not exactly 
repeat dust concentrations from day to 
day to allow repeated RAM-1 readings to 
be made of identical dust concentrations. 



where y = RAM-1 reading, mg/m^ , 

X = mean gravimetric reading, 
mg/m3 , 

m = regression slope, 

and b = y axis intercept, mg/m^, 

TABLE 1. - Regression statistics 



RAM-1 



Coal 



1 



ARD 



Limestone 





REGRESSION SLOPES 






A 


0.90 

0) 

1.08 


2.14 
2.50 
1.73 


0.94 
0) 
.82 


0.75 
0) 
.95 


0.99 


B 


1.08 


C 


0) 





CORRELATION COEFFICIENTS 




A 


0.71 
0) 
.81 


0.89 
.95 
.93 


0.95 
0) 
.97 


0.82 
0) 
.88 


0.96 


B 


.96 


C 


(1) 



STANDARD DEVIATION OF REGRESSION 



A 


0.49 
0) 
.42 


0.77 
.66 
.48 


0.65 
(1) 
.50 


0.66 
0) 
.64 


0.49 


B 


.52 


C 


0) 



^Unit was not available. 



EFFECT OF PARTICLE CHARACTERISTICS 

The next question addressed is whether 
or not the RAM-1 responds differently to 
different dusts. To do so, the values 
for the slope of the regression of the 
RAM-1 versus gravimetric readings for 
each type of dust are compared. The 
slope characterizes the instrximent re- 
sponse well if the response is linear and 
if the correlation between RAM-1 readings 
and gravimetric measurements is high. It 
has already been demonstrated that the 
RAM-1 response is linear with mass con- 
centration; now the degree of correlation 
between the RAM-1 and gravimetric read- 
ings will be examined. 

Table 1 shows the values for the 
slopes, correlation coefficients, and 
standard deviation of regressions (stan- 
dard error of estimate) from the linear 
regression analyses for figures 5 through 
9. The regression equation is 



y = mx + b 



(4) 



The sample correlation coefficient (r) 
is an estimate of the true correla- 
tion coefficient (p) which is the degree 
of association between the y and x val- 
ues in a statistical relationship. The 
values of r in table 1 range from 0.71 
(a fair correlation) to 0.97 (a high cor- 
relation) , with a test case average of 
r = 0.89. In general then, the light- 
scattering signal from the RAM-1 corre- 
lates well with mass concentration — at 
least when dust parameters such as size, 
index of refraction, density, and shape 
are held constant. 

Because the RAM-1 response is linear 
and correlates well with gravimetric mea- 
surements of mass concentration, the 
slopes of the regression lines can be 
reasonably interpreted as the response of 
the RAM-1 to a particular dust. The 
slopes can be used to compare the RAM-1 
response to different test dusts. Table 
1 lists the regression slopes for each 
RAM-1 unit and for each test dust. Ex- 
cept for coal 2 cases, the regression 



11 



slopes center around 0.9, ranging from 
0.75 to 1.08. The regression slopes for 
the tests with coal 2, however, are no- 
ticeably higher — roughly by a factor of 
2. This difference is shown in figure 
10. 

Because of data scatter, regression 
analysis provides only an estimate of the 
true regression; that is, some uncertain- 
ty exists about the true slope and inter- 
cept of the regression line. The uncer- 
tainty is a function of the extent of the 
scatter. 

Each regression in figure 10 has an 
associated uncertainty. Does the data 
scatter around the regression lines in 
figure 10 nullify the apparent differ- 
ences among the slopes? The standard es- 
timate of error (Sy^j^) for the sany>le is 
a measure of the data scatter around the 
calculated regression line. If the cal- 
culated regression is a good estimate of 
the true relationship between the RAM-1 
values and the mass concentration values, 
then Sy^j^ is a good estimate of Oy.^* the 
true standard estimate of error. By def- 
inition, 68 pet of data pairs should lie 
within the values of ±ay,x» 

In figure 10, the gray shaded intervals 
surrounding the regression lines repre- 
sent S„ ^. For test dusts other than 
y .X 

coal 2, individual Sy^^^ intervals overlap 
to a great extent; the larger shaded area 
in figure 10, about the regression lines, 
represent the outermost bounds of all 
the overlapping S ^^ intervals. The Sy^^ 
interval for coal 2 does not overlap the 
others for concentrations greater than 1 
or 2 mg/m^. Although more rigorous tests 
could show the same results mathematical- 
ly, this simple analysis graphically il- 
lustrates that the response of the RAM-1 
to coal 2 is different than its response 
to the other dusts. The results for coal 
2 are real: The behavior was verified by 
repeating tests. 

Efforts were then made to discover why 
the RAM-1 responded so differently to 
coal 2 than to coal 1 and the other 
dusts. What was different about coal 2? 
The characteristics to be considered 



are size, density, shape, and surface 
properties. 

Size or Density 

Table 2 lists size distribution and 
density data for the test dusts. The 
values for density (pq) were not mea- 
sured, but were taken from the litera- 
ture. The mass median aerodynamic diam- 
eters (MMAD) and geometric standard 



4- 



e 

CD 

■z. 
Q 3 

< 

LU 

a: 
< 

b 2 



< 







1 \ 


/ ' ' .yj 


A I 


y\f 


I 


// *^ 


/ 
~ / 


//x- 


/ 
/ 




/ 
/ 


//fy^ 


/ 
/ 


//^ 


/ A 


"^ 


I /A 


r KEY 


- / /W 


Coal 1 


t/// 


Coal 2 


»p<y/ 


......ARD 


4Mf/ 


Limestone 1 - 


m/ 


Limestone 2 


m 




M 1 1 


1 1 







Coal 2 

ARD 
Limestone 



^ I 2 3 4 5 

GRAVIMETRIC MEAN, mg/m^ 

FIGURE 10. - Responseof RAM-1 to coal 1, coal 2, 
limestone 1, limestone 1, and Arizona Road Dust. 
Shaded intervals show Sy.x* 



12 



deviations (^g) were measured using an 
Anderson Mark III stack sampler cascade 
impactor. These table values for parti- 
cle size represent the average of vari- 
ous measurements made throughout the test 
program. 

TABLE 2. - Dust parameters 



Dust 


MMAD,1 
ym 


ag,2 
pm 


g/cm^ 


Coal 1 


3.5 
3.6 
1.3 
1.6 
.5 


2.0 
2.7 
2.8 
3.8 
5.4 


1.3 


Coal 2 


1.3 


ARD 


2.6 


Limestone 1.* 


2.7 


Limestone 2 


2.7 



^Mass median aerodynamic diameter, 
^Geometric standard deviation. 
3 Particle density. 

To determine the MMAD and Og , the cxjmu- 
lative mass percentage collected on each 
impactor stage was calculated. These 
values were plotted on log-probit scale 
paper against the aerodynamic cut diam- 
eter for each stage. The data in all 
cases approximated a straight line imply- 
ing that the particle sizes were lognor- 
mally distributed. A computer was used 
to plot the size data, to calculate a 
linear regression, and to identify the 
MMAD and Og based on the regression 
analysis. 



From table 2, it can be seen that the 
MMAD for coal 1 and 2 are nearly the 
same, but that Og is 2.0 for coal 1 and 
2.7 for coal 2. It was thought perhaps 
this difference in size distribution pa- 
rameters was responsible for the large 
difference in RAM- 1 response between coal 
1 and coal 2. If so, a proportional 
response difference between limestone 1 
and limestone 2^ could also be expected. 

The size distribution differences be- 
tween limestone 1 (MMAD = 1.6 ym, Og 
= 3.8) and limestone 2 (MMAD =0.5 ym, Og 
= 5.4) are greater than between coal 1 

^Limestone 2 was generated by passing 
limestone 1 through a cyclone size clas- 
sifier (D50 = 3.5 ym) before 
into the test chamber. 



and coal 2. Accordingly, the RAM-1 re- 
sponse difference between limestone 1 and 
limestone 2 would be expected to be even 
greater than between coal 1 and coal 2. 
In fact, however, the regression slope is 
0.75 for limestone 1 and 0.99 for lime- 
stone 2 — much less than the response dif- 
ference between coal 1 and coal 2. In- 
deed, particle size may be somewhat 
responsible for these differences in re- 
sponse; however, particle size cannot 
solely account for the large response 
difference between coal 1 and coal 2. 

Shape Factor 

Assuming that particle size parameters 
are not the dominant cause of the RAM-1 
response difference between coal 1 and 
coal 2, other possibilities were investi- 
gated. Figures 11 and 12 are scanning 
electron microscope micrographs of coal 1 
and coal 2. The micrographs were com- 
pared and no marked differences in parti- 
cle shape were seen. A stereoviewer was 
used to look at the dust particles in 
three dimensions to see if perhaps the 
particles in one sample tended to be 




feeding it 



FIGURE 11.- Scanning electron microscope 
micrograph of coal 1 (X 300). 



13 




FIGURE 12. - Scanning electron microscope 
micrograph of coal 2 (X 300). 

flatter than the particles in the other 
sample. Here again, the two coal dust 
samples were indistinguishable. 

Surface Properties 

Finally, the coal dust samples were ex- 
amined with an optical microscope at a 
magnification of X 105 using white light 
to illuminate the sample. Two individual 
observers described coal 2 as "sparkling" 
much more than coal 1; that is, the sur- 
faces of coal 2 appeared to be more high- 
ly reflective. 

One hypothesis is that some type of 
surface degradation had occurred during 
the 1- to 2-yr storage time for coal 1, 
dulling the surface of the particles. 
Coal 2 on the other hand, had been fresh- 
ly prepared. If this hypothesis is true, 
dust from freshly mined coal should be 
more like coal 2. 

Although surface reflectance appears 
to be a dominant cause of the difference 
between the RAM-1 responses for coal 1 
and coal 2, particle size cannot be 



eliminated as a potential cause of re- 
sponse variations. Response character- 
istics are likely to be a function of 
the combination of size, surface reflec- 
tance, and index of refraction. For ex- 
ample, the effect of size may be greater 
for highly reflective particles such 
as coal; whereas the effect of size may 
be diminished for more nonreflective par- 
ticles such as ARD or limestone. To 
resolve such issues is difficult, large- 
ly because of the difficulty in repeat- 
edly generating particles with known 
characteristics . 

While the observed phenomenon is diffi- 
cult to explain, the important facts are 
that the response of the RAM-1 to differ- 
ent dusts can indeed be different and 
that the RAM-1 response is constant (pre- 
cise and linear with mass concentration) 
for consistent dust. Thus, the RAM-1, as 
with all light-scattering devices, must 
be calibrated with the dust to be mea- 
sured if the user wishes to accurately 
estimate mass concentrations. 

Calibration 

To calibrate any instrument, one must 
assimie that a strong, well-defined corre- 
lation exists between the phenomenon to 
be measured and the response of the in- 
strument. The evidence presented to this 
point indicates that if the parameters of 
the test dust are held constant, the 
RAM-1 response is fairly well represented 
by a linear function of mass concentra- 
tion. Therefore, the RAM-1 can be cali- 
brated to indicate mass concentrations of 
dusts as long as the particle character- 
istics pertinent to light scattering do 
not change. Recommendations on the cali- 
bration of the RAM-1 are discussed in 
appendix B. 

RAM-1 PRECISION 

The final question to be addressed in 
this report is "How reproducibly can the 
RAM-1, once calibrated, predict the true 
mass concentration?" To answer this 
question, the 95-pct confidence interval 
about the linear regression line will be 



14 



determined and expressed as a percentage 
of the value of dust concentration pre- 
dicted by the regression equation for a 
selected RAM-1 reading. 

But first, the regression values listed 
in table 1 result from a regression of y 
on x; that is, RAM-1 values on gravimet- 
ric values. The form of the regression 

equation is 



y(RAM-l) - ™^(grav) "*" ^* 



(5) 



This equation is used to estimate what 
the RAM-1 might read when exposed to a 
particular mass concentration (x) . Nor- 
mally, however, a user obtains a reading 
with the RAM-1 and from that estimates 
the true mass concentration. 

Generally, when a relationship is sta- 
tistical in nature, a regression equation 
cannot be algebraically inverted. Spe- 
cifically, since the correlation coeffi- 
cient |r| ^1 for the relationship be- 
tween the RAM-1 reading and the mass 
concentration, 

y(RAM-i) " ^. 



"(grav) 



m 



(6) 



Thus, the regression analysis must be 
redone after reversing the x and y val- 
ues. That way, the regression analysis 
will minimize the error in the predicted 
value of mass concentration for a partic- 
ular reading on the RAM-1. The new re- 
gression equation is in the form 

y(grav) = ^' ^(R/KtA-}) + b' , (7) 
where m' ^ m 



and 
unless 



b' ?t b 

Irl = Ir'l = 1. 



the true relationship between the mass 
concentration and the RAM-1 response. 

The regressions were recalculated using 
equation 7. The 95-pct confidence inter- 
vals (W, ) for the regression lines were 
calculated using the equation 



W, = /2F s 



n "*" S, 



'XX 



where 



s = 



Syy - (Sj(y) / S j^ ^ 



1/2 



T 1/2 



(8) 



n - 2 

Syy = I (yi - y)^ 

Sxx = I (X, - x)2, 

Sxy = I [(x, - x)(y, - y)]. 



n ^ 



i » 



and 



n = number of points, 

X| = value of x at which W| is 
calculated, 

F = table value for percentiles 
of F distribution. 



Note: For the tests, F = Fq 95 (2, n 
- 2). 

Then, for each RAM-1 unit and dust 
type, the 95-pct confidence interval at 
the predicted value of 2.0 mg/m^ was 
determined as a percentage of that value 
(2.0). The intervals were examined at 
the predicted value of 2.0 mg/m^ because 
the value 2.0 is fairly well centered in 
the range of data used in the regression 
analysis. Those percentages are listed 
in table 3, 



The mass concentration predicted by the 
new regression equation at a particular 
RAM-1 reading is the mean of a normal 
distribution of possible mass concentra- 
tion values. That mean represents the 
true mass concentration only to the ex- 
tent that the regression line represents 



An example to illustrate the signifi- 
cance of these results is as follows: 
Based on the value in table 3 for coal 2, 
whenever RAM-1, unit A, reads 4.35 mg/m^, 
the true mass concentration calculated 
using the regression equation will be 2.0 
mg/m^ ±7.1 pet, 95 pet of the time. Note 



15 



TABLE 3. - Ninety-five-percent 
confidence intervals at Yc^ 
= 2,0 mg/m^ , percent 



RAM-1 unit 



A 

B 

C 

Av.' 



Coal 



16.6 

(2) 

12.1 

36 



7.1 
5.7 
6.4 



33 



ARD 



27.2 

(2) 

24.0 

28 



Limestone 



34.2 

(2) 

25.2 

20 



25.4 
24.8 
(2) 

11 



^Predicted RAM-1 reading from regres- 
sion analysis. 

^Unit was not available. 

^Not all RAM-1 units were available for 
each test; therefore, these values are 
the average number of tests for each dust 
type. 

that one could vary the gain of the in- 
strviment so that the regression slope m' 
= 1. If in that case the interval W^ 
remained the same, then when RAM-1, unit 
A, read 2.0, the true mass concentration 



would be 2.0 mg/m^ ±7.1 pet, 95 times out 
of 100. 

The analysis using equation 8 results 
in an interval (W^ ) about predicted y 
values (gravimetric readings) that is 
valid for any and all x values (RAM-1 
readings) in the range of data collected. 
Other methods are available to estimate 
the confidence interval for a single 
point on the line, but intervals calcu- 
lated for a single point on a line are 
valid only for that point. The W^ 's ob- 
tained from equation 8 are conservative 
estimate of the confidence of prediction 
because they are larger than intervals 
calculated about y for a single value of 
X by the ratio /2F/t. As stated in ref- 
erence 13, "This wider interval (W,) is 
the 'price' we pay for making joint 
statements about y for any number of or 
for all of the x values , rather than the 
y for a single x," 



CONCLUSIONS 



For specific dusts whose particle char- 
acteristics do not change significantly — 

1. The RAM-1 response is linear with 
mass concentration. Thus, if dust parti- 
cle characteristics are expected to be 
reasonably constant, the RAM-1 can confi- 
dently be used to evaluate dust control 
techniques where relative rather than 
absolute measurements are sufficient. 

2. RAM-1 readings correlate well with 
mass concentration. Thus, a functional 
relationship can be derived between the 
RAM-1 output and mass concentration to 
calibrate the instrument for a particular 
dust. One might also adjust the gain of 
the instrument so that the RAM-1 directly 
indicates the mass concentration, but the 
gain setting would be different for dif- 
ferent dusts. 

3. The precision of the instrument 
(that is, its ability to reproducibly es- 
timate the true mass concentration) is 
good, probably on the order of presently 
used gravimetric devices. Note that this 



precision estimate is for cumulative 
RAM-1 measurements made over periods 
greater than 4 h. The 95-pct confidence 
interval at 2,0 mg/m^ ranged from about 
±6 pet for coal 2 to around ±30 pet for 
limestone 1, The percentages calculated 
for limestone 1 and 2 were higher because 
fewer data points were available for the 
statistical analysis. 

The estimates of precision presented 
here are conservative. Some of the er- 
rors attributed to the RAM-1 actually re- 
sulted from random dust losses in tubing 
(see appendix A). When the RAM-1 is used 
with the cyclone attached immediately be- 
fore the sensing chamber (no tubing) , the 
error caused by random dust losses in the 
tubing would be eliminated. 

4. The response of the RAM-1, as with 
all light-scattering devices, is depen- 
dent on the characteristics of the dust 
being measured. It is emphasized that if 
the users wish to estimate the true mass 
concentration, they must calibrate the 
instrument with the dust to be measured. 



16 



RECOMMENDATIONS 



The RAM-1 should be field tested in a 
wide variety of environments. The RAM-1 
performs predictably well when exposed to 
laboratory generated dusts, but the re- 
sponse is affected by particle character- 
istics. Sufficient data regarding the 
range and frequency of particle charac- 
teristic variation in field environments 
are not available. Such data would allow 
the user to predict the effect on RAM-1 
measurements of mass concentrations. 



Work done by Tomb and Gero (14) indi- 
cated that in underground coal mines , 
the precision of long-term (>4 h) RAM-1 
measurements is equal to or better than 
that of gravimetric personal samplers, 
and that once calibrated, the instrument 
read within ±10 pet of the gravimetrical- 
ly determined mass concentration. Work 
of that type should continue to determine 
the limitations of the instrument when 
used in the field. 



REFERENCES 



1. Breur, H. Das Feins taub-Streu- 
lichtphotometer TM Digital (The Fine-Dust 
Light Scattering Photometer TM Digital). 
Staub-Reinhalt Luft, v. 36, Jan. 1976, 
pp. 6-10. 

2. Williams, K. L. , and G. H. 
Schnakenberg, Jr. Direct Measurement of 
Respirable Dust. Paper in Proceedings of 
the Fifth WVU Conference on Coal Mine 
Elect rot echnology , July 30-31, August 1, 
1980 (WV Univ., Morgantown, WV) . BuMines 
OFR 82-81, 1980, pp. 18-1—18-13. 

3. Marple, V. A., B. Y. H. Liu, and 
K. L. Rubow. A Dust Generator for Labo- 
ratory Use. Am. Ind. Hyg. Assoc. J., v. 
39, 1978, pp. 26-32. 

4. Blackman, M. W. , and M. Lippmann. 
Performance Characteristics of the Multi- 
cyclone Aerosol Sampler. Am. Ind. Hyg. 
Assoc. J., V. 35, 1974, pp. 311-326. 

5. Caplan, K. J., L. J. Doemeny, and 
S. D. Sorenson. Performance Characteris- 
tics of the 10-mm Respirable Mass Sam- 
pler: Pt. I — Monodisperse Studies. Am. 
Ind. Hyg, Assoc. J., v. 38, 1977, pp. 83- 
95. 

6. . Performance Characteris- 
tics of the 10-mm Respirable Mass Sam- 
pler: Pt. Il-Coal Dust Studies. Am. 
Ind. Hyg. Assoc. J., v. 38, 1977, 
pp. 162-173. 

7. Corn, M. , and W. Bell, A Tech- 
nique for Construction of Predictable 
Low-Capacity Critical Orifices, Am, Ind, 



Hyg, Assoc, J,, V, 24, 1963, pp, 502- 
504, 

8. Aerosol Technology Committee. 
Guide to Respirable Mass Sampling. Am. 
Ind. Hyg. Assoc. J., v. 31, 1970, 
pp. 133-137. 

9. Lilienfeld, P. Improved Light 
Scattering Dust Monitor (contract 
H0377092, GCA Corp.). BuMines OFR 90-79, 
1979, 44 pp.; NTIS PB 299 938. 

10. Tomb, T. F., H. N. Treaftis, and 

A. J. Gero. Instantaneous Dust Exposure 
Monitors. Environ. Int., v, 5, 1981, 
pp, 85-96, 

11. Crow, E. L. , F. A. Davis, and 
M. W. Maxfield. Statistics Manual. 
Dover Publications, Inc., New York, 1960, 
288 pp. 

12. Mie, G. Considerations on the 
Optics of Turbid Media, Especially Col- 
loidal Metal Solutions. Ann. Phys. (Leip- 
zig), V. 25, 1908, pp. 376-445. 

13. Natrella, M. G. Experimental Sta- 
tistics. U.S. Department of Commerce, 
National Bureau of Standards, Handbook, 
1963, p. 91. 

14. Tomb, T. F,, and A. J. Gero. De- 
velopment of a Machine-Mounted Respirable 
Coal Mine Dust Monitor, Ch, in Aerosols 
in the Mining and Industrial Work En- 
vironments, ed, by V. A. Marple and 

B. Y. H, Liu, Ann Arbor Science, Ann Ar- 
bor, MI, V. 3, 1983, pp. 647-663. 



17 



APPENDIX A. —DUST LOSSES IN TUBING 



The standard procedure used for deter- 
mining the response of the RAM-1 to vari- 
ous dusts involved comparing RAM-1 mea- 
surements with gravimetric measurements. 
For the gravimetric samples, the respira- 
ble dust that exits the cyclone deposits 
immediately on a filter. For the RAM-1, 



however, respirable dust that exits the 
cyclone must travel through approximately 
3 ft of flexible tubing before reaching 
the light-scattering sensing chamber. 
The following tests were performed to ex- 
amine possible dust losses in the flexi- 
ble tubing during transport. 



EXPERIMENTAL PROCEDURE 



Of the 10 gravimetric samplers normally 
used to determine the mean mass concen- 
tration of dust inside the dust chamber, 
five were modified to be like the ones 
used in the RAM-1 sampling train (see 
figure 4 in the main text). The filter 
cassette immediately atop the cyclone was 
removed and the flexible tube was at- 
tached directly to the exit port of the 
cyclone. The filter was reinserted into 
the sampling line approximately 3 ft 
downstream. In these modified gravimet- 
ric systems, therefore, respirable dust 
traveled through the same length of 



flexible tubing as in the RAM-1 sampling 
train before being collected on the 
filter. 

Measurements made using the standard 
gravimetric sampling trains were desig- 
nated as "inside" measurements since the 
filter cassette was located inside the 
Lippmann-type sampling arrangement with 
the cyclone. Measurements made using the 
modified gravimetric sampling trains were 
designated as "outside" measurements 
since the filter cassette was located 
outside the test chamber. 



EFFECT ON BIAS 



The mean of the five outside gravimet- 
ric measurements were compared with the 
mean of the five inside gravimetric mea- 
surements. The scatter plot and linear 
regression are shown in figure A-1. At 



to 




F 




^ 




en 

E 


4 


r- 




lU 




Q 




!^ 


?> 


(- 




3 




O 




fh 




2 


2 


< 


liJ 




2 




O 


I 


a: 




t- 




LlI 




s 




> 





< 




a: 




o 





Out = 1.09 (ln)-0.23 
r= 0.90 

- Sy.x = 0.28 




± 



first glance, the slope of 1.09 is dis- 
turbing since this would imply that dust 
is not lost in the tubing, but rather is 
created! However, a regression line is 
only an estimate of the true relation- 
ship. One can test the significance of 
the results in the following way (7^).^ 

A null hypothesis that the true slope 
of regression (M) is equal to 1 (that is, 
no dust is lost in the tubing) is stated. 
A value, t, can be calculated using the 
following equation: 



t = 



m - M 



(A-1) 



where m 



1 2 3 4 

GRAVIMETRIC MEAN, INSIDE, mg/m^ 

FIGURE A-1,- Comparison of respirable dust con- 
centrations measured with filters inside the dust cham- 
ber (no tubing) with concentrations measured with fil- 
ters outside the dust chamber (approximately 3 ft of 
tubing). 



and 



M = 



S„ = 



the slope of the regression 
estimate, 

the slope of the true 
regression, 

estimated standard deviation 
of the value m. 



^ Underlined numbers in parentheses re- 
fer to items in the list of references 
preceding this appendix. 



18 



From the regression analysis, m = 1,09, 
Sn, = 0.086, and M = 1 is selected arbi- 
trarily. Substituting into equation A-1, 
t = 1.05. This t value is compared with 
a t distribution table value t(Qj/2, n-2) 
where a is the significance level and 
n is the number of tests. In the re- 
ported case, a = 0.05 and n = 39 were 
selected. The table value t(o,o25 37) 
= 2.03. Now since t > t(Qt/2, n-2) > the 
null hypothesis cannot be rejected at 
the 0.05 significance level; that is, 
there is no statistically based reason 
to conclude that M is not equal to 1. 
Based on the data, it cannot be stated 
with any certainty that dust is lost in 
the tubing. 



by 



The confidence interval for M is given 
M = m ± t(a/2, n-2)(Sm). (A-2) 



Substituting into equation A-2, M = 1.09 
±0,17 or 0,91 < M < 1,26, 



From the physics of the situation, M 
> 1 would not be expected because the 
tubing cannot create dust. However, dust 
losses in the tubing could be expected to 
be less than 10 pet, 95 times out of 100, 

The preceding discussion deals with 
dust losses, or biases (systematic er- 
rors) in the comparison of RAM-1 readings 
with gravimetric readings. Such biases 
could be important when calibrating the 
RAM-1 to indicate mass concentration as 
determined by gravimetric devices. How- 
ever, since the tests were to examine re- 
sponse behavior, this bias, if consist- 
ent, is not significant. What must be 
determined, however, is whether trans- 
porting the dust through the tubing in- 
troduces more random error. Since the 
same lengths of tubing are used in the 
RAM-1 sampling trains, any random error 
introduced by dust losses in the tubing 
would appear as less precision in the 
RAM-1 measurement. 



EFFECT ON PRECISION 



In figure A-2, the amount of data scat- 
ter about the estimated regression line 
is higher when the RAM-1 measurements are 
compared with gravimetric measurements 
made through a length of tubing (outside) 
than when the RAM-1 measurements are com- 
pared with gravimetric measurements made 
immediately after the cyclone (inside). 
Sy^x is an estimate of the data scatter 
about the regression line. For RAM-1, 
unit C, Sy^x increased from 0.42 to 0.55, 
or by 31 pet. For RAM-1, unit A, Sy^x 
increased from 0.49 to 0,58, or by 18 
pet. On the average, the random error 
introduced by drawing the dust through 



the tubing caused a 24-pct increase in 
the value of Sy^x* Therefore, one can 
safely assume that some of the scatter 
about estimated regression lines for 
RAM-1 measurements (all drawn through 
tubing) compared with gravimetric mea- 
surements made inside the dust chamber 
(no tubing) is due to random dust losses 
in the tubing to the RAM-1, in addition 
to that random measurement error inherent 
in the RAM-1. If this indeed is true, 
the estimates of the ability of the RAM-1 
to reproducibly predict the true mass 
concentration are conservative. 



19 



CO 

E 

o 

z 

o 

< 
0:4 
< 

I- 



1 1 1 T" 1 ^ 

RAM-1 = 0.90 gravimetric + 0.34 ° 


1 

y^ 


y 


- r = 07l 


° 0^ 


y^ 




_Sy.x=0.49 


^^ 

„ S'^ 








o°X^ 









yb 






~ 


/^ 






- 






- 


/ 


Q^ 




- 


1 


1 1 1 1 


A 

1 


- 




to 

e 

o» 

E 

cT 
z 

Q 
< 

UJ 

o 







I 1 1 1 1 1 1 
RAM-l=l.08gravimetric+0.55 ^ 
r = 0.8l ° ^oy^ 


/ 


-Q =042 ° ^^ 


- 


° y/^° 


- 


B 

1 1 1 1 1 1 1 


- 



=> 4 



< 



1 1 1 1 1 1 r 

RAM-l=0.72gravimetric+l.53 ^ ° 
r=0.64 o o 

■Sy.x=0.55 ° 

o 
o 




± 



± 



_l_ 



3 4 I 2 3^ 

GRAVIMETRIC MEAN, mg/m3 

FIGURE A-2. - Comparison of respirable coal dust concentrations measured by RAM-1 with concen- 
trations measured gravimetrically inside {A^ B) and outside {C, D) the dust chamber. 



20 



APPENDIX B. —RECOMMENDATIONS ON THE CALIBRATION OF THE RAM-1 



Previous Bureau work showed that the 
zero and gain of the RAM-1 are exception- 
ally stable (9^). These tests have shown 
that for a given test dust, the RAM-1 re- 
sponse is linear and correlates well with 
mass concentration. Based on these find- 
ings , one can reasonably expect that — 

1, The RAM-1 can be calibrated to in- 
dicate respirable mass concentrations di- 
rectly in milligrams per cubic meter. 

2. The RAM-1 must be calibrated for 
each type of dust to be measured. 



Once calibrated, the reliability of sub- 
sequent measurements will then depend on 
the variability of the properties of the 
particles in the dust cloud. 

To calibrate the RAM-1, the user must 
compare RAM-1 and gravimetric measure- 
ments as was done in these tests. The 
user can then either adjust the gain of 
the RAM-1 so that the instrument displays 
the proper concentration values , or 
develop a calibration curve to convert 
displayed values to the true mass 
concentration. 



TUBING LOSSES 



In either case, the user should be 
aware that some dust losses can occur 
if the dust is transported through long 
lengths of flexible tubing (see appen- 
dix A) . Since the intent of this eval- 
uation was only to observe response 
behavior, systematic losses were not 
important. However, if the user wishes 
to use the RAM-1 to obtain absolute val- 
ues of dust concentrations, he or she 
should calibrate the instrument in the 



configuration in which it will later be 
used. In other words, if the application 
of the instrument will require that the 
sampled dust be drawn through tubing to 
the RAM-1 sensor, then the instrument 
should be calibrated using tubing of the 
same approximate length and material. 
That procedure will at least reduce bias 
in the measurements although the preci- 
sion may still be measurably reduced. 



DEVELOPING A CALIBRATION CURVE 



In examining the behavior of the RAM-1, 
target concentrations of 1, 2, 4, and 10 
mg/m^ were arbitrarily selected. The 
conclusions were based on regressions for 
data in that concentration range. Ex- 
trapolating these regressions to very 
high concentrations could conceivably 
lead to very large errors. If the user 
wishes to make measurements at such high 
concentrations, he or she should make 
some comparative measurements at those 
levels during the calibration procedure. 

Since the RAM-1 was tested at concen- 
trations above nominally 1 mg/m^ , little 
or no meaning was attached to the y in- 
tercept of the regression equations. A 
functional relationship to be used for 
calibration, however, would be greatly 
simplified if the y intercept were zero. 
Therefore, the user, when developing a 
calibration curve, may wish to perform 
the regression analysis to force the 
line through the origin. Many statistics 
books, including Natrella (13) , offer 
procedures for such a regression. 



Before using such a procedure, however, 
one should be sure that the RAM-1 does 
indeed indicate zero in dust-free air. 
Manufacturer data and earlier Bureau data 
(9^) indicate that the RAM-1 's zero indi- 
cation, once adjusted in a dust-free 
environment, is not affected by tempera- 
ture, humidity, etc. Background scatter- 
ing resulting from dust contamination of 
the optics would cause a zero shift. 
However, it was found that the clean-air 
sheath (9^) over the optical surfaces suc- 
cessfully prevents deposition of dust. A 
properly adjusted RAM-1 should, there- 
fore, read mg/m^ when no dust is pres- 
ent in the air. 

Although forcing the regression through 
zero can be justified, the user should 
nevertheless make comparison measurements 
at concentrations near mg/m^ . These 
measurements would (a) establish linear- 
ity in the low concentration range and 
(b) estimate the precision of the RAM-1 
at low concentrations. 

INT.-BU.OF MINES, PGH., PA. 27347 



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